So I recently bought a 2003 about two weeks ago and immediately the day after, I had to replace the alternator. The car seemed to run fine for the next week and a half but then after I bought a brand new battery, the alternator cut out completely. It kicked back on about halfway home but then stopped working ever since.
When I decided to take out the brand new alternator and have it tested (and it tested good), I noticed that the top-most bolt (which I'm assuming is the bolt that grounds the alternator) was barely even tightened down. But the most alarming thing was after I got the whole thing out, the positive post was heavily corroded, like somebody just took a bite out of the post, there's no way after just two weeks would something look like that.
So I got the new alternator replaced and made sure this time that everything is tightened down correctly on the way in. But the same thing is already starting to happen, the first time I started the car it held steady at around 14.9v but after refilling the radiator and taking the car on a drive, the voltage started at around 14.9 but was dropping steadily over time and now reads around 12.3v-13v idle and 14v whenever I start to drive.
Now since I haven't had a working alternator so far, is that normal? Does your Z's fluctuate like that? Also, do you think that maybe there is a short affecting the positive terminal? I don't know of anything else that could cause that kind of corrosion.

Don't know if any of this helps but I've noticed the car has an aftermarket radiator, APS Plenum, UR underdrive crank pulley, aftermarket tailights, and I can tell that the car had an aftermarket stereo at one time because there are still wires for the amp and sub, but it's been since taken out.

EDIT: I can confirm that the reason the voltage doesn't drop when I first start the car is because the car idles a little bit higher to warm up, but once the car is satisfied and idles at normal rate, the voltage drops.

 

Make sure your battery terminals are tight and clean

 

Aftermarket tail lights..LED type...can cause a host electrical problems due to resistance issues that are out of spec.

 

The terminals look like they've just been replaced.

I thought this too, but I disconnected the taillights and still had the problem, not unless the problem is only fixed through connecting stock taillights.

I've tried the fuse and it's perfectly fine, the battery is brand new, and the alternator is brand new, and the upgraded ground wires look fine. I really can't think of what else it could be, I'm gonna try and see if I have a short somewhere when I get time.

 

Sorry for double post but do you guys think it might be the underdrive crank pulley? Because the voltage only drops when the car is idling around 500rpm, but if I blip it up to about 800rpm, everything's fine. So maybe the pulley is rotating the alternator too slow at idle?

 

The car should idle at 750 rpm, not 500.

 

Sorry I was going from memory, now that I look at it it is actually just under 750. But I still really think it's the pulley, I'mma try and find a stock size one.

 

Ignore this post, I kinda jumped the gun on the poblem